1. Field of the Invention
The present invention relates to an electronic control circuit for an automatic speed changer, and particularly relates to the current control of the linear solenoid for controlling the oil pressure in the oil pressure circuit thereof.
2. Prior Art
In recent years, in an electronic control type automatic speed changer (in the following, referred to as A/T), in controlling the oil pressure supplied to the clutch and brake, which are the speed changing mechanisms, it is known to use an electronic control device which employs an actuator such as a linear solenoid or a duty solenoid. Since the driving signals of the linear solenoid or duty solenoid were PWM signals, and the current flow was also comparatively large, the signals were subject to noise which resulted in errors in the performance of the electronic control device itself and in the signals to the electronic control device.
Therefore, as a countermeasure, the electronic circuit for the automatic speed changer (A/T) is designed so that, in correspondence to needs, the ground signals of the electronic control device are divided into a ground for the current flow of the linear solenoid, referred to as PE (power ground) below, and a ground for the current flow of the microcomputer in the electronic control device and interface circuit, referred to as SE (signal ground) below.
An example of the technology described above is shown in FIG. 5 which is a block diagram of the current control system of the linear solenoid of a conventional electronic control type speed changer. FIG. 6 is a partial circuit diagram of the current control system of FIG. 5.
As shown in FIGS. 5 and 6, since the electronic control device regulates the oil pressure of the automatic speed changer in accordance with the degree of throttle opening, responsive to signals from the throttle sensor 11, it controls the current of the linear solenoid 12. Also, as a noise countermeasure, it has the so-called PE for grounding the current passed through the linear solenoid and the so-called SE for grounding the current of the interface circuit of the microcomputer separately.
The electronic control device 1 is equipped with a microcomputer 2 comprising a linear solenoid current setting unit for receiving the signals from the throttle opening degree judging unit 3, a PWM (pulse width modulation) signal output unit 5, a monitor current comparing correction unit 6, and a monitor current judging unit 7. Further, it is equipped with a solenoid driving circuit 8 for driving the linear solenoid 12, a solenoid current monitor circuit 9, and a current monitor device (current monitor use resistor) 10.
However, when an electric potential difference is generated between PE and SE, the result is that the line pressure of the A/T does not coincide with the set value.
For example, in the case when PE is higher than SE by .DELTA.V (V), the potential difference of the current monitor 10 for PE by the linear solenoid current is denoted as V.sub.F, the ground of the solenoid current monitor circuit 9 becomes V.sub.F, =V.sub.F +.DELTA.V due to SE, when it is seen from the solenoid current monitor circuit 9. This potential difference represents a current which is larger than the current actually flowing through the linear solenoid 12. Since the above-described feed back control acts in accordance with such erroneous signals as described above, the actual value of the current flowing in the linear solenoid 12 becomes lower than the set value.
In the case when SE is higher than PE for .DELTA.V (V), the potential difference becomes V.sub.F, =V.sub.F -.DELTA.V when seen from the solenoid current monitor circuit 9. This potential difference represents a current which is less than the current actually flowing in the linear solenoid 12. Therefore, since the above-described feed back control acts in accordance with such erroneous signals as described above, the actual value of the current flowing in the linear solenoid 12 becomes higher than the set value.